Light-emitting semiconductor diodes (LED's) are increasingly being used as light sources in numerous applications due to their many advantages over traditional light sources. LED's generally consume significantly less power than incandescent and other light sources, require a low voltage to operate, are resistant to mechanical shock, require low maintenance, and generate minimal heat when operating. As a result, they are displacing incandescent and other light sources in many uses and have found applications in such disparate areas as traffic signals, large area displays (including video displays), interior and exterior lighting, cellular telephone displays, automotive displays, and flashlights.
LED's are typically used in such applications as components in assemblies. LED assemblies comprise a housing partially surrounding at least one LED and an electrical connection between the diode and an electrical circuit. The assembly may further comprise a lens that is adhered to the housing and that fully or partially covers the LED and serves to focus the light emitted by the LED.
It would be desirable to make LED housings from polymeric materials, as such materials may be injection molded and offer considerable design flexibility. However, useful polymeric compositions would preferably satisfy a number of conditions. Since many LED assemblies are attached to circuits boards using reflow oven welding processes that operate at elevated temperatures, useful compositions would be sufficiently heat resistant to withstand the welding conditions and minimal surface blistering of the housing during the welding process. Useful compositions would further preferably exhibit good whiteness/reflectivity to maximize the amount of light reflected by the housing, have good ultraviolet light resistance, good long-term resistance to the operating temperatures of the LED assembly, and have good adhesion to any lens material used. The poly(1,4-cyclohexanedimethanol terephthalate) compositions used in the present invention satisfy the foregoing requirements.
WO 03/085029 discloses a polyamide resin composition useful in the production of light-emitting diode reflectors. However, polyamides often do not have good color retention upon exposure to ultraviolet light or heat.